Which one has better FE? CRX Si or DX? - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

Reply
 
Thread Tools Display Modes
 
Old 07-01-2008, 09:33 PM   #1
Registered Member
 
Join Date: Jun 2008
Posts: 24
Country: United States
Which one has better FE? CRX Si or DX?

Here's the deal. I owned a 91 CRX DX automatic(I know it sucks) and have a chance to pick up a 91 CRX Si parts car. I was thinking about just swapping the Si engine and transmission over to my car, definitely for the 5-speed. Just want to know if I should keep the DX motor with the transmission, or just go for the whole shebang.

Might just hunt for an HF transmission for the longer gears.
__________________

__________________
Honda Civic VX Video Review
XyKo is offline   Reply With Quote
Old 07-02-2008, 04:07 AM   #2
Registered Member
 
Join Date: Jun 2008
Posts: 8
Country: United States
The Si gears are really tight on the CRX (I have a bone stock 88 si) and at 60 mph you are already at 3000+ rpm's. A DX manual has longer gears (different final drive ratio) and would be better in FE in my opinion, and a HF even more.
__________________

rankink is offline   Reply With Quote
Old 07-02-2008, 04:32 AM   #3
Registered Member
 
Join Date: Oct 2007
Posts: 298
Country: United States
Quote:
Originally Posted by rankink View Post
The Si gears are really tight on the CRX (I have a bone stock 88 si) and at 60 mph you are already at 3000+ rpm's. A DX manual has longer gears (different final drive ratio) and would be better in FE in my opinion, and a HF even more.
I disagree about the HF transmission. I actually tried using an HF transmission with a DX motor that originally had an SI transmission. And flat-out MASSACRED my fuel economy. Given the fact that the SI motor is actually more aggressively cammed than the DX motor, I would expect things to be even WORSE with that motor. If you want the HF transmission to help you with mileage, it MUST be used with a motor that is designed to take advantage of that tall gearing. So your best bet would be either a complete HF drivetrain swap OR (better) a D15Z1 swap with the HF transmission. Otherwise, if you want to use the SI engine, just use it with the SI transmission. You might be able to pick up SOME economy with DX manual transmission on the SI engine (or DEFINITELY with your stock DX motor). And, given that you could probably get one for free, why not experiement?
StorminMatt is offline   Reply With Quote
Old 07-02-2008, 04:50 AM   #4
Registered Member
 
Join Date: Apr 2008
Posts: 137
Country: United States
Quote:
Originally Posted by StorminMatt View Post
I actually tried using an HF transmission with a DX motor that originally had an SI transmission. And flat-out MASSACRED my fuel economy.
That will kill your in town driving but the hwy mpg should have gone up. What guys on the CRX forum do is swap the HF 5th into the Si trans. They keep the Si final drive ratio so 5th overall is still not as tall as in the HF, but it is a big help on the highway. It maintains good gearing for around town but on the highway you have a sort of "super" overdrive. Most of them report getting over 40mpg highway with that hybrid transmission.
__________________


opelgt73 is offline   Reply With Quote
Old 07-02-2008, 08:41 AM   #5
Registered Member
 
GasSavers_bobski's Avatar
 
Join Date: May 2007
Posts: 463
Country: United States
Quote:
Originally Posted by StorminMatt View Post
If you want the HF transmission to help you with mileage, it MUST be used with a motor that is designed to take advantage of that tall gearing.
Yeah, it's a matter of where the engine makes it's torque. The D15B2 (DX motor) makes peak torque at 4500 RPM, the D15B6 (HF motor) makes peak torque at 2800 RPM. The HF transmission's gearing keeps engine RPM in the 'B6's range of usable torque. If you bolt the 'B2 to the HF transmission, the engine will be outside its usable range and so will struggle to produce enough torque to maintain highway speeds. It's that struggling that kills the FE.
The D15Z1 is spec'd as having peak torque at 4800 RPM, but it's a VTEC-E motor, so it changes cam profiles in the middle of its RPM range. It uses an HF-like low-end torque profile at low RPM, but switches to a more DX-like profile above that. The result being a more or less flat torque curve, but it hits a little peak at 4800, so that's where the peak torque spec lies.
GasSavers_bobski is offline   Reply With Quote
Old 07-02-2008, 05:12 PM   #6
Registered Member
 
Join Date: Oct 2007
Posts: 298
Country: United States
Quote:
Originally Posted by opelgt73 View Post
That will kill your in town driving but the hwy mpg should have gone up. What guys on the CRX forum do is swap the HF 5th into the Si trans. They keep the Si final drive ratio so 5th overall is still not as tall as in the HF, but it is a big help on the highway. It maintains good gearing for around town but on the highway you have a sort of "super" overdrive. Most of them report getting over 40mpg highway with that hybrid transmission.
Highway MPG didn't go up. It went down. As for swapping an HF fifth gear in an SI transmission, this only makes fifth as tall as a stock DX transmission - a far cry from fifth in the HF.
StorminMatt is offline   Reply With Quote
Old 07-02-2008, 06:21 PM   #7
Registered Member
 
Join Date: Apr 2008
Posts: 137
Country: United States
Quote:
Originally Posted by StorminMatt View Post
Highway MPG didn't go up. It went down.
Then something else was wrong
Quote:
Originally Posted by StorminMatt View Post
As for swapping an HF fifth gear in an SI transmission, this only makes fifth as tall as a stock DX transmission - a far cry from fifth in the HF.
But still much better than the Si.

Try these two threads

http://crxcommunity.com/viewtopic.php?t=16427
http://crxcommunity.com/viewtopic.php?t=15913
__________________


opelgt73 is offline   Reply With Quote
Old 07-02-2008, 06:36 PM   #8
Registered Member
 
GasSavers_bobski's Avatar
 
Join Date: May 2007
Posts: 463
Country: United States
Quote:
Originally Posted by StorminMatt View Post
As for swapping an HF fifth gear in an SI transmission, this only makes fifth as tall as a stock DX transmission
Well, it would be a good combination then.
The DX 5th gear/final drive combo is very similar to the DX auto's 4th gear/final drive combo in terms of RPM-to-speed. I've gotten milage in the low 40s with both the D16A6 (Si engine) and D15B2 (DX, as I said earlier) engines, and nearly 46 mpg with a D16Z6 though the DX auto transmission.


Top gear ratio * final drive:

Si: 0.771 * 4.250 = 3.277
DX manual: 0.771 * 3.888 = 2.998
DX auto: 0.780 * 3.933 = 3.068
HF federal: 0.694 * 2.954 = 2.050
HF Cali: 0.694 * 3.250 = 2.256

Si/HF hybrid: 0.694 * 4.250 = 2.949

So the Si/HF combo is geared just a hair slower than a DX.
GasSavers_bobski is offline   Reply With Quote
Old 07-02-2008, 08:16 PM   #9
Registered Member
 
Join Date: Jun 2006
Posts: 125
Country: United States
I'd look for an hf and sell your dx
white90crxhf is offline   Reply With Quote
Old 07-02-2008, 08:59 PM   #10
Registered Member
 
Join Date: Apr 2008
Posts: 137
Country: United States
Quote:
Originally Posted by white90crxhf View Post
I'd look for an hf and sell your dx
They're still out there.... I am picking up a rust free HF tomorrow morning
__________________

__________________


opelgt73 is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
starter issues again lunarhighway General Maintenance and Repair 9 03-10-2008 04:20 AM
HydroCarbons CATs O2 Sensors JanGeo General Fuel Topics 0 03-06-2008 08:13 AM
Kind of a kewl product...tape DarbyWalters General Fuel Topics 0 11-17-2007 08:09 PM
Swift 1.3 GT seats posted in Classifieds ...Anyone ever check the classifieds? Brian D. General Fuel Topics 4 10-01-2007 05:55 PM
Magnets for blocking popimp General Fuel Topics 9 06-22-2007 07:41 AM

Powered by vBadvanced CMPS v3.2.3


All times are GMT -8. The time now is 01:41 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.